I'm no expert, but this is how I understand it:
- you pay a *one time fee* when your Executive spreads a corporation to a foreign city (much like a Missionary does when spreading religion, except that does not cost any money). This fee varies, in my games, it usually ends up costing around 100 gold.
- the "-3 coins" (or: "-x") you see is the cost which would be incurred due to "corporation" payments; these are *per turn*, and are much like maintenance; in fact, they are listed in the "maintenance" section of your city screen. They can be reduced by Courthouses and can vary depending on the civic (e.g. free markets cost less, environmentalism more).
- the "+1 coins" (or: "+x") you see is the income you receive for the number of resources. I am away from game and civloped right now, but unless I am mistaken, the Jewelry corporation gives its main bonus in gold. Hence, you probably have very few resources for that corporation if you are only getting +1 coin, possibly only 1 or 2 resources. This can increase dramatically when you get more resources.
Hence, it will not "always" turn a profit; you will run on a deficit if, for example, you have very few resources, have no courthouses in the cities to which it is spread, and run an expensive civic (like environmentalism) -- plus the cost of actually spreading the corporation (one time fee). It will frequently turn a profit, however, if you have a decent number of resources and run a more lucrative civ.
Does that help?